1. Field of the Invention
The present invention relates to a PON (Passive Optical Network) type optical communication network, and more particularly to an optical communication network using a communication system combining optical time division multiplexing and optical wave division multiplexing systems.
2. Description of Related Art
As an optical communication network, PON, for example, is known. PON is a system for sharing one optical fiber by a plurality of subscribers (that is, homes which join the service) by branching an optical communication path by a coupler. PON is used by such a service as FTTH (Fiber To The Home). As an optical communication network based on PON, GE (Gigabit Either)-PON, A (Asynchronous transfer mode)-PON, B (Broadband)-PON, G (Gigabit)-PON, WDM (Wavelength Division Multiplexed)-PON and DWA (Dynamic Wavelength Allocation)-PON, for instance, are known. Here, GE-PON is standardized as IEEE 802.3ah, A-PON and B-PON are defined as ITU-T Recommendation G.983 series and G-PON is defined as ITU-T recommendation G.984 series.
GE-PON is a system for sharing a 1 gigabit/second transmission band (that is, communication speed) by 16 or more (e.g. 32) subscribers. As a document to disclosure GE-PON, the following Document 1, for example, is known.
Document 1: Hiromichi Shinohara: “Current status and future perspective of high-speed optical access system”, O Plus E, Japan, The Shin-Gizyutsu Communications Co., Inc., January 2003, Vol. 25, No. 1, pp. 49-55
FIG. 13 is a conceptual diagram depicting a general configuration of GE-PON, and is substantially the same as FIG. 5 of Document 1. As FIG. 13 shows, the terminal device at the telephone station side, that is the OLT (Optical Line Terminal) 1301 is connected to 16 or more terminal devices at the subscriber side, that is, the ONUs (Optical Network Unit) 1304, via the optical fibers 1303 branched by the optical coupler 1302. The OLT 1301 is connected to a basic network 1305, such as a local IP (Internet Protocol) network or the Internet, and each ONU 1304 is connected to the communication terminal 1306, that is a personal computer.
As FIG. 13 shows, in the case of data transmission in the down direction, the Ethernet™ frames addressed to any ONU 1304 are time-division multiplexed and sent from the OLT 1301 to each ONU 1304. Each ONU 1304 extracts only the Ethernet™ frames addressed to itself from the receive data, and discards the other Ethernet™ frames. The optical wavelength in the down direction is 1480-1500 nm.
In the case of data transmission in the up direction, Ethernet™ frames are sent from each ONU 1304 to OLT 1301. At this time, each Ethernet™ frame can be time-division multiplexed using the optical coupler 1304 by appropriately adjusting the transmission timing of each ONU 1304. The optical wavelength in the up direction is 1260-1360 nm.
In GE-PON, DBA (Dynamic Bandwidth Assignment) is performed to improve data transmission efficiency in the up direction. DBA is a technology to change the transmission band (that is, communication speed) to be assigned to ONUs 1304 according to the number of ONUs 1304 in the transmission operation. A wider band is assigned to an ONU 1304 during transmission as the number of ONUs 1304, which perform the transmission operation simultaneously, is less. DBAs are employed by above A-PON, B-PON and G-PON.
The WDM-PON is a system of sharing an optical fiber by assigning a different optical length to each ONU. As a document to disclose WDM-PON, the following Document 2, for example, is known.
Document 2: Ivan Kaminov: “Optical Fiber Telecommunications IV B Systems and Impairments”, Academic Press, USA, March 2002, pp. 480-481
FIG. 14 is a conceptual drawing depicting an example of a general configuration of WDM-PON, and is a drawing substantially the same as FIG. 10.8a in Document 2.
As FIG. 14 shows, the OLT 1411 and the wavelength router 1412 are connected by one optical fiber 1413, and the wavelength router 1412 is connected to a plurality of ONUs 1414 respectively. Each ONU 1414 uses a different optical wavelength from one another, and the optical wavelengths used in the down direction and in the up direction are also different. Therefore if the number of ONUs 1414 is “n”, then the number of the types of optical wavelengths used for data transmission is “2n”.
In the WDM-PON in FIG. 14, a two-core optical fiber may be used instead of a one-core optical fiber 1413 (see FIG. 10.8b-e in Document 2). If a two-core optical fiber is used, an optical multiplexer is used for the up direction, and an optical demultiplexer is used for the down direction instead of the wavelength router 1412 (see FIG. 10.8b in Document 2). By using a two-core optical fiber, each ONU 1414 can use a same wavelength for the up direction and down direction (see FIG. 10.8c in Document 2). Also in the case of WDM-PON in FIG. 14, a spectrum slicing or time-division multiplexing can be used for transmission in the up direction (see FIG. 10.8d, e in Document 2).
DWA-PON is a technology to dynamically assign wavelengths to be used in a system where a plurality of PONs are integrated. DWA-PON is disclosed, for example, in the following Document 3. In the case of the optical communication network shown in FIG. 1 of Document 3, four types of wavelengths are dynamically assigned to 4×16 ONUs of four systems of PONs.
Document 3: Yu-Li Hsuch, et al: “Success-DWA: A Highly Scalable and Cost-Effective Optical Access Network”, IEEE Optical Communications, August 2004, S 24-S30
GE-PON (see FIG. 13) uses the time-division multiplexing system as described above. Therefore in GE-PON, a transmitter and a receiver in OLT need not be installed in each corresponding ONU, but only one transmitter and one receiver are required. Therefore in GE-PON, the equipment scale of the telephone station can be decreased, which is an advantage.
However in GE-PON where one wavelength is assigned to all the subscribers (e.g. 32), the communication speed per subscriber decreases as the number of subscribers (that is the number of ONUs) who simultaneously communicate increases. On the other hand, increasing the communication speed of one wavelength has a limitation, due to such a reason as receive sensitivity. So the transmission assurance band of GE-PON is narrow, which is a disadvantage.
On the contrary, WDM-PON (see FIG. 14) uses different wavelength channels depending on the ONU. Therefore, transmission bands do not change according to the change in number of subscribers who perform communication simultaneously. As a result, sufficiently large transmission bands can be assured.
However in WDM-PON, the wavelength to be used is different depending on the ONU to be accommodated. Therefore the same number of transmitters and receivers as the number of ONUs must be installed in OLT. As a result, in WDM-PON, the equipment in the telephone station reaches a large scale and is expensive, which is a disadvantage.
Also in the case of WDM-PON, the wavelength is fixed for each subscriber. Therefore the optical telephone station must manage information on the wavelength assigned to each subscriber for each PON. This increases the management cost of WDM-PON.
WDM-PON uses an AWG (Arrayed Wave Guide) type optical multiplexer/demultiplexer. In AWG, the dependence of wavelength on temperature is high (0.011 nm/° C.), and 1° C. or less of precise temperature management, for example, is required. This makes the management cost of WDM-PON high. Whereas if an interference film type WDM (Wavelength Division Multiplexing) filter is used, then temperature management is unnecessary. However the number of wavelengths that can be realized by interference film type WDM filter technologies is limited. WDM-PON sometimes needs 32 or more branches. This means that it is virtually impossible to perform wavelength demultiplexing equal as WDM-PON by using interference film type WDM filter technologies.
DWA-PON can flexibly distribute the transmission band among a plurality of PONs by dynamically assigning the optical wavelengths. Therefore in the case of DWA-PON, the transmission bands can be substantially spread while suppressing the increase of equipment cost and management cost.
However in DWA-PON, only one type of wavelength can be simultaneously assigned to one ONU, which restricts flexibility and transmission efficiency when the transmission band is allocated.